Radioactively labeled 60-70S RNA of various MMTVs (GR, RIII, Balb/cfC3H) were obtained from cell cultures of mammary tumors and shown to be free of contaminating murine leukemia viruses by a variety of techniques. These RNA's were used in competition hybridization experiments to determine the relationship of the genomes of various MMTVs to one another. 60-70S RNAs isolated from the MMTVs from primary mammary tumor cultures of RIII, C3H, and GR mice were shown to share at least 95 percent nucleic acid sequence homology. A difference was observed, however, between the MMTV derived from early C3H mammary tumors and the MMTV derived from the C3Hf mammary tumor. Direct molecular hybridization experiments of these 3H 60-70S RNAs, to the DNAs of distribution of MMTV proviral DNA. The proteins of MMTV from various mouse strains have also been compared. The 60-70S RNA of MPMV was compared to the RNA of other oncornavirus isolated by competitive molecular hybridization. No homology was detected between MPMV RNA and various MMTVs, the type B BudR induced guinea pig virus, or several rodent, feline, and primate type C viruses. The RNA from morphologically similar isolates from primate tissues, however, displayed complete homology with MPV RNA.